What takes so much space in my external disk?

My external disk (LaCie) is full! When I look at the individual folders within my disk, I see that only little space is occupied by visible documents. So I displayed the hidden files and found small files and directories that all seem to take very little memory. Looks like about 400GB is occupied by some unknown stuff. However there is a directory called .trashes.

When I click on GetInfo, here is what I see

And I can wait for a long time but never get an estimate of the size of this directory. I tried to estimate its size via the Terminal:

$ du .Trashes
du: .Trashes: Permission denied

I tried to delete .Trashes but I get this window popping up

How can I free space on my external disk?

I am on OSX Yosemite, Version 10.10.2.